Amongst the known signals of the time-multiplexed type are those of the AIS system. The AIS (Automatic Identification System) is a public communications system between ships which allows collisions between these ships to be limited and which allows the maritime traffic in view from the coasts to be monitored thanks to coastal stations that listen to the communications. This system advantageously implements satellites for relaying the communications over a wide area. In order to allow the satellite AIS to have an acceptable performance with respect to the performance demands for maritime safety, the highest possible number of tools must be implemented to utilize the colliding signals in order to extract the message from at least one of them.
The circumstances in which signals can collide are many-fold. The colliding signals come from different SO-TDMA (Self Organizing Time Division Multiple Access) cells. The differences in frequency (Doppler), power (distance/free-space loss, antenna gain) and synchronization (distance/propagation time) between these various signals are variable which can allow their discrimination.
Solutions do exist (simple demodulator, SIC or “Sequential Interference Canceller”) that provide access to at least one of the colliding signals, in the case where there are a sufficient number of parameters discriminating between them: difference in synchronization, difference in carrier or difference in power.
When the said parameters of two signals are too close, the performance in terms of bit error rate is seriously degraded. In particular, in the worst case of collision between two signals (i.e. signals received at the same power and with identical Doppler frequencies), access to the two signals becomes impossible.
The article “Co-Channel Demodulation for Continuous-Phase Modulated Signals, Peter A. Murphy & Gary E. Ford, IEEE proceedings” discloses a method, the “Joint Maximum Likelihood Sequence Estimation (J-MLSE)”, providing possibilities for demodulation of two signals in certain collision circumstances, encountered in cellular communications systems. Thus, this method is applicable in the case of two signals with the same carrier frequency and whose digital contents are synchronized when they are received. This type of method therefore seems to be valid, but in circumstances that are too restrictive, which will not be encountered in other systems, such as in particular the satellite AIS.